1. Technical Field
The present invention relates to a semiconductor device and a method for manufacturing thereof.
2. Related Art
It is known that a reverse current is flowed as shown in FIG. 11, when an operation of a PN junction diode is switched from a forward operation to an off state [see, for example, “Physics of Semiconductor Device”, authored by S. M. Sze, passages 2. 6. 1 (Transient Behavior), 2. 7. 5 (Fast-Recovery Diode), and 2. 7. 6 (Charge-Storage Diode)]. This is a phenomenon, which is occurred by reducing a concentration of a minority carrier, which has been accumulated in a diffusion layer in the forward operation of the diode, to an original concentration in a switch to an off state. The minority carrier is recombined in a diffusion layer to be dissipated, or, when an electric current path is present, the minority carrier is released toward outside and is observed as a reverse current.
If a concentration in the diffusion layer is lower as in a diode having higher breakdown voltage in particular, a recombination rate of the minority carrier is also reduced, causing an increase in time for flowing reverse current therethrough (recovering time). Consequently, a defective situation may be arisen in the operation due to an unexpected reverse current in some circuit. Further, in bipolar transistors utilizing action of minority carrier, a problem of deteriorating the switching characteristic is also occurred by an increase of a recovering time.
An use of a fast recovery diode is proposed as a method for reducing such recovery time in “Physics of Semiconductor Device”, authored by S. M. Sze, passages 2. 6. 1 (Transient Behavior), 2. 7. 5 (Fast-Recovery Diode), and 2. 7. 6 (Charge-Storage Diode). Further, one of typical fast recovery diodes is a Schottky diode. While the recovery time can be ignored by employing the Schottky diode due to smaller amount of accumulated minority carrier, the use of the Schottky diode may cause problems of larger leakage current and difficulty in providing higher voltage-resistance. Further, proposed another solution for reducing the recovery time is to reduce lifetime of a carrier by diffusing gold (Au) or the like to accelerate an annihilation thereof. However, when such solution is applied to integrated circuits, a problem of occurring non-negligible influences to other devices is caused.
A method for solving these problems is disclosed in “Reduction in Minority Carrier Storage Effect by Fluorine Ion Implantation Damage”, IEEE Transactions on Electron Devices, Vol. ED-25, No. 7, July 1978, pp. 772-778. Such method will be is described in reference to FIG. 12. In FIG. 12, an n-type region 102 is formed in a p-type substrate 101, and a p-type diffusion layer 103 is formed in the n-type region 102. A diode is composed of the n-type region 102 and the p-type diffusion layer 103. Further, an n-type diffusion layer 106 functioning as a contact layer of such region is formed in the n-type region 102. An interlayer insulating film 104 is formed on the p-type substrate 101. An opening 105 is formed on a section of the interlayer insulating film 104 located above the p-type diffusion layer 103.
The above-described article in IEEE Transactions on Electron Devices describes that fluorine ion is injected into a lower portion of the p-type diffusion layer 103 through the opening 105 of the interlayer insulating film 104, and the injection damage occurred in such injection promotes an increased rate of recombination of minority carrier. In FIG. 12, a region where fluorine is injected is schematically illustrated with x marks.
In addition to the above described two literatures, other prior art document related to the present invention is Japanese Patent Laid-Open No. H10-74,959 (1998).
However, ion implantation is achieved only in the lower portion of the p-type diffusion layer 103 by the method described in reference to FIG. 12. Hence, the recombination rate of minority carrier accumulated between the p-type diffusion layer 103 and the n-type diffusion layer 106 cannot be enhanced.